PRoD #2: Flipping hot Jupiters

Press Release of the Day is a short, pithy, accurate summary of the day’s weirdest, most interesting, or most-fun-to-write-about science press release.

Image courtesy of CIERA

With the awesomely nonsensical headline/catchphrase/band name “Flipping hot Jupiters,” astrophysicists at Northwestern University have announced a model of planet formation that explains why some extrasolar planets appear to orbit their stars in the ‘wrong’ direction.

Among the many surprises caused by the discovery of planets orbiting other stars is the “hot Jupiter” phenomenon, in which a massive Jupiter-like gas giant orbits very close to its star. Even more puzzling, some hot Jupiters appear to orbit ‘backward’ relative to the direction their star spins around its own axis. That challenges current models of planet formation, which are based on our solar system—in which all planets orbit with the Sun’s spin.

The Northwestern model shows how interactions between two massive planets could put one of them into a backwards or “retrograde” orbit. The trick is that the planet does not reverse direction—instead, the planet’s orbit flips over.

A lot of things have to line up for this to occur: the two planets’ orbits have to start off tilted relative to each other; the less massive planet must be orbiting closer to the star; and the timing of their orbits must bring them into the right place at the right time. When conditions are right, the heavier planet periodically tugs on the other, increasing the tilt of its orbit very slightly. Add up enough little tilts, though, and the orbit flips like a pancake in slow motion.